This invention relates generally to RF power transistors, and more particularly the invention relates to combining output matching and RF and video bypass circuitry in a packaged RF power transistor.
RF power transistors providing 10 watts or more of power are typically packaged as a discrete device, as shown generally at 10 in FIG. 1A and schematically in FIG. 1B. The packaged transistor (FET or bipolar) normally includes an input matching circuit 12 connecting input lead 14 to a gate of a FET (or base of a bipolar transistor) and an output matching circuit 16 connecting output lead 18 to the drain of a FET (or collector or emitter of a bipolar transistor). Normally the source of the FET is grounded.
The packaged transistor is typically mounted on a printed circuit board 20 as shown in FIG. 2. Also mounted on the printed circuit board are matching circuits 22, 24 and a bias/RF diplexer 26 which connect the transistor output to an RF output. A DC power supply is connected to the transistor output lead through a bias line RF and video bypass circuit 28, diplexer 26, and matching circuit 22. This dual function of the printed circuit board circuitry presents a desired impedance transformation at the carrier frequency, and provides a means of injection of DC bias current to the device through the diplexer. The circuitry must also properly bypass the power supply connection at RF and video frequencies. The distance of the circuitry from the packaged transistor is a limitation in this conventional approach. This separation introduces significant electrical delay, and the inherent characteristics of capacitors used in printed circuit board circuits causes an unavoidable limitation in bias circuit video bandwidth.
In accordance with the invention, the output matching circuit and diplexer along with the RF and video bypassing network are integrated with the power transistor within the transistor package. This places the circuitry closer to the transistor and increases the power supply bypassing video bandwith. Further, the linearity (fidelity) of the power transistor circuit is improved relative to conventional devices for wideband signals.
In implementing the invention, discrete multilayer capacitors of the RF and video bypassing network can be connected in parallel with a capacitor (with optimal properties at the carrier frequency) of the output matching circuit in an integral arrangement with wire bonding providing inductive elements of the circuitry. Typically a wirebond is set to provide a shunt inductive reactance that is equal and opposite to the reactance of the transistor parasitic output capacitance. Additional capacitance for the RF output can be provided also with the RF output providing a bias input or functioning with a separate bias input.
The invention and objects and features thereof will be more readily apparent from the following detailed description and dependent claims when taken with the drawings.